Extinguishing

There are three ways to extinguish a fire:

cool it down. This is why usually water is poured onto a fire, but
this is not generally advisable.

deprive it of oxygen. This is a popular way e.g. with gravure printing
machines, where after some alarm time carbon dioxide gas is released into
the production room. The alarm time is necessary because the room has to
be evacuated from people. Note that some fires, e.g. burning light metal
dusts (oh yes, they can !), will react with carbon dioxide, making the
fire even worse.

interfere with flame chemistry. Flame chemistry is essentially radical
chemistry. Radicals produced by thermal decay of extinguishing agents will
interfere with the flame reaction and stop it. The most popular compounds
of this kind were the "halons", but these now have been banned
because of their detrimental effects to the earth's ozone layer.

Types of reaction

Flamesare exothermic chemical oxidation reactions in the gaseous state. The
oxidant usually is atmospheric oxygen, which is supplied by diffusion from
the outside of the flame. Hence, the oxidation proceeds relatively slow.

Explosionsoccur if the flammable substance has been premixed with the oxidant.
It is possible in the gaseous, liquid, or solid state. The chemical reaction
proceeds much more violently, as temperature rises more quickly. Propagation
speeds are typically around 100 m/s and may reach the velocity of sound
(333 m/s). The initial pressure may increase tenfold.

Detonationsare even faster and more violent than explosions. Their speed is above
the velocity of sound and typically reaches several thousand meters per
second; military explosives are as fast as 12,000 m/s. Energy tranfer is
not by conductance of heat, but by shock. The initial pressure may increase
thousandfold or more.

Explosion limitsIn the gas phase above a liquid there is always some vapour of the
liquid. The pressure exercised by this vapour onto the walls of a closed
vessel is called the vapour pressure. When the vapour pressure is equal
to or higher than the ambient pressure, the liquid boils.

The vapour pressure depends on the temperature of the liquid.
The Clausius-Clapeyron equation says how:

Dalton's law says that the partial pressures of gases can be added to
give the overall pressure in a gas mixture.
Hence, the concentration of a certain (flammable) gas in the air can be
estimated from its vapour pressure. It can only be estimated and not calculated
exactly, as there are no ideal gases.

Note that explosion limits, or flammable limits, are temperature-dependent,
especially when given in [g/m³].

Example (from a German regulation, substance not given):

LEL (20 °C 290
K)

40 g/m³

LEL (310 °C 580
K)

24 g/m³

This difference is easily understandable with regard to the universal gas
law,

pV = nRT,

which says 2 T ==>n/2;
the deviation from this result is mainly due to the dilution of the reactive
gases.

Dust explosions

Even combustible solids, when finely dispersed and raised, may explode.
Such events are called dust explosions. Dust explosions usually cause severe
damage.

Examples

(from the German "Handbook of Disasters")

In 1967 at Hawthorne, New Jersey, USA, starch was roasted in a chemical
plant. Several dust explosions, caused by a steam-heated roaster, killed
eleven people und destroyed several buildings. Estimated cost: $ 3,650,000.

In 1979 in Bremen, Germany, there was a fire near to a flour mill, causing
a dust explosion there. 14 were killed and 17 injured. Estimated cost:
more than 10,000,000 DM. As compared to military explosions, this disaster
was calculated to be equivalent to the detonation of 20 tons of TNT, the
standard military explosive.

Test of a dust explosion

Destruction caused by dust explosion (1979)

(Source: Fonds der Chemischen Industrie, Germany;

imageseries "Farbstoffe und Pigmente")

Paper dust, finely dispersed, may cause dust explosions. Here are the
results of two tests, performed with paper dust from dust filters.

Inertization means to reduce the oxygen
concentration to a safe level.

The relevant parameter is the limiting oxygen concentration, LOC,
. It has
to be noted, however, thatdepends
on the other gases present.

Examples

[% by moles]

inert gas

Hydrogen

5,0

5,0

Petrol

11.8

14.5

Methane

12,1

14,6

Benzene

11,2

13,9

These figures depend on pressure and temperature.

The concentrations in a system may be such that any addition of fuel
cannot result in an explosive mixture (partial inertization) or such that
any addition of air cannot make up a dangerous mixture (total inertization).

Diagrams like the triangular plot below sum up the explosive behaviour
of flammable substances. The explosive region is shaded.

The arrows at point T explains how the concentrations are plotted.

Point D: LOC,

An ignition source must exceed some minimum energy to set off an explosion.

The relevant figure is the minimum ignition energy, MIE. This is the
smallest amount of energy, which is sufficient to ignite the most readily
ignitable fuel-air mixture, when stored in a capacitor and released in
a spark discharge.

Examples for MIEs

[mJ]

Hydrogen

0,011

Ethylene

0,07

Propane

0,25

i-Propanol

0,65

Aluminum dust

2

Methyl cellulose

1000

Examples for frequent
ignition energies

[mJ]

Electrostatic discharge

4

Grinding machine

100

Welding spark

10000

Electrical equipment is considered safe, if none of the following quantities
can be exceeded:

1.2 V, 0.1 A, 20 µJ, 25 mW.

Where possible, it is an engineer's task to design containers, reactors,
etc., in such a way that they can withstand the pressures built up if the
content goes off. This is what the chemical industry mainly relies on.
Safety valves or rupture disks combined with pressure relief pipes leading
outward or - preferably - to safety tanks will direct the gases in a preset
way and thus protect the rest of the installation or even the building.
It must be made sure that the hot gases that might escape do not cause
further harm.

In the construction of buildings where explosions might occur, it is
important to create sufficient pressure relief areas, such as pressure
relief windows or walls. These are lightly built structures that will collapse
if under small pressure, thus saving people in the building and possibly
the building itself. Be careful with pressure relief roofs: these tend
to eventually fall down again.